I have a 2011 nissan frontier 4.0 with a automatic transmission. I do not do any towing with this truck. When should I change the fluid? And should I have the fluid change or the transmission flushed?

Regular service > drain fluid - replace filter - refill with proper fluid. Best not to flush a transmission which has several miles on it - it can result In particulates being transferred into critical transmission fluid passages. I personally do not recommend it .... However 'some' do - they usually have the flush machine to pay for as well ..!